1. Field of the Invention
The present invention relates to synchronous digital transmission.
2. Description of the Prior Art
For this type of transmission, which often features transformer coupled lines, use is generally made of a signal having no DC or low-frequency components and made up of a synchronous stream of multivalent, usually ternary, symbols coding the binary data to be transmitted. Being conveyed by channels of necessarily limited bandwidth, these symbols are subjected to attenuation and distortion which render them unrecognizable after a particular transmission distance.
The forms of distortion having the highest nuisance value include intersymbol interference. An attempt is usually made to avoid such interference by applying criteria established by Nyquist, that is to say be conferring on the equalized transmission channel a frequency response such that the sum of the amplitudes of two frequencies symmetrical to the half-frequency of the symbols is constant. The most commonly used of these frequency responses is the so-called raised cosine response which is characterized by the band percentage utilized in excess relative to the symbols half-frequency f.sub.1, known as the "roll-off" factor, the value of which is often chosen as 0.7 to limit individual deformation of the symbols (aperture of the eye diagram) to an acceptable level.
An alternative to eliminating intersymbol interference is to control it. This is the object of the so-called partial response technique as described, for example, in U.S. Pat. No. 3,388,330. This technique entails modeling the response of the transmission channel in such a way that its impulse response is spread over an integer multiple of the symbol duration T and so that the signal at the receiving end for a sequence of symbols results from a linear combination of waveforms in [sin (.pi.t/.sub.T)]/(.pi.t/.sub.T) each having for its origin one of the symbols transmitted. This technique makes it possible to limit strictly the bandwidth of the transmission channel in the high-frequency domain to the symbol half-frequency f.sub.1 and therefore to minimize thermal noise relative to transmission without intersymbol interference, at the cost of an increase in the number of levels to be distinguished at the receiving end.
For some type channels, transmission with partial response type controlled intersymbol interference is more advantageous than transmission without intersymbol interference as it improves the signal-to-noise ratio, the advantage of reducing the thermal noise outweighing the disadvantage of increasing the number of levels at the receiving end.
There are various classes of partial response depending on the linear combination of symbols obtained at the receiving end, each of these classes being subdivided according to the number of symbols in the combination.
Class 1, type n=2 is one of those most commonly employed. It is defined by a transfer characteristic for the equalized transmission channel defined by the equation: EQU H(f)=cos .pi.Tf
which produces at the receiving end for each symbol transmitted a signal resulting from the superposition of two consecutive waveforms in sin [(.pi.t/.sub.T)]/(.pi.t/.sub.T) offset by one period T and each assigned a multiplication coefficient corresponding to the level of the original symbol. On sampling a received symbol corresponding to the transmission of ternary symbols with levels 0, .+-.1 there are obtained five possible values of amplitude denoted 0, .+-.1, .+-.2 and corresponding to addition of the level of the ternary symbol transmitted to that of the symbol which immediately preceded it. Detection consists in the converse operation, that is to say in subtracting the level of the previously detected symbol on sampling the signal received. Utilizing the level of the previously detected symbol, this process is subject to error propagation which is, however, limited by the fact that the extreme sampling levels of the received signal correspond to a transmitted symbol having an extreme value of the same sign as these levels and can only result from the superposition of two consecutive symbols of this type.
An object of the present invention is a logic circuit which implements this detection operation at high speed and with limited propagation of errors and which is easy to integrate.